This application claims the priority of 198 37 340.6-21, filed Aug. 18, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a steering system in a vehicle which is equipped with a traffic-lane following device, and more particularly to a steering system having a steering handle, for example a steering hand wheel, for introducing a manual steering force into the steering system. A mechanical steering run mechanically couples the steering handle to the steerable vehicle wheels, and a servomotor is coupled to the steering run in order to operate the steering of the vehicle steered wheels. A servovalve is arranged in the steering run and has a first control part (which is associated with a section on the steering handle side of the steering run) as well as a second control part(which is associated with a section on the vehicle steered wheel side of the steering run. The servovalve operates the servomotor as a function of the relative movements between the control parts. An actuating drive, is coupled to the section on the steering handle side of the steering run for introducing a mechanical steering force into the steering system. A regulating and control arrangement which uses a sensor system to determine a required steering angle value which allows the vehicle to follow a predetermined traffic lane, determines an actual steering angle value by way of a steering angle sensor and operates the actuating drive as a function of a comparison of the required and actual values of the steering angles in order to introduce a steering force.
A conventional servo-assisted steering system has a steering handle, for example a steering hand wheel, with which the driver manually introduces a steering force into the steering system. The steering handle is connected to a mechanical steering run, which couples the steering handle to steerable vehicle wheels. The servo-assisted steering system also has a servovalve which is normally arranged in the steering run and has a first control part on the input side and a second control part on the output side.
The servovalve in the known system separates a section on the steering handle side in the steering run, which is coupled to the first control part, on the input side, from a section on the vehicle steered wheel side, which is coupled to the second control part, on the output side. Relative movements, in particular relative rotations, between the control parts of the servovalve then operate a servomotor, which is coupled to the section on the vehicle steered wheel side of the steering run, and thus to the vehicle steered wheels. If the servomotor is a hydraulic motor, hydraulic fluid pressure is applied in an appropriate manner to the servomotor via the servovalve.
In order to avoid any small steering forces on the steering handle from leading to a movement of the control parts in the servovalve and thus to steering angle changes on the wheels, the control parts of the servovalve are coupled to one another via a spring system, in particular via a torsion rod or a C spring. Thereby, the spring system prestresses the control parts to a normal position. In order that the driver can input a steering command into the steering system, the driver must accordingly manually apply a steering force to overcome the restoring force of the spring system in the servovalve. This measure is also used, in particular, to give the driver a sensation of the forces acting on the vehicle steered wheels in order to improve driving safety.
Modern vehicles may be equipped with a traffic-lane following device. Such a traffic-lane following device may, for example, generate required steering angle values with an appropriate sensor system. Such values, when they are applied to the vehicle steered wheels, allow the vehicle to follow a predetermined traffic lane. For example, the traffic-lane following device may be equipped with a camera which detects, for example, a strip on the side of the roadway and produces required steering angle values as a function of the profile of this side strip.
In order for the vehicle to automatically adopt and maintain the required steering angle value via the traffic-lane following device, the steering system with such a traffic-lane following device has an actuating drive which is coupled to the section on the steering handle side of the steering run and introduces steering forces in order to set the desired steering angle in the steering run. The mechanical steering force produced by the actuating drive thereby supports or replaces the manual steering force applied by the driver during normal operation, or even counteracts it. Depending on the safety philosophy adopted by the vehicle manufacturer, the driver can even remove his or her hands from the steering handle when in the traffic-lane following mode.
In order to regulate and control the actuating drive, a regulating and control arrangement uses signals generated by the sensor system of the traffic-lane following device to determine a required steering angle value and also uses a steering angle sensor to determine an actual steering angle value. The regulating and control arrangement then carries out a comparison of the required and actual values of the steering angle and operates the actuating drive in an appropriate manner in order to introduce into the steering system or into the steering run the steering force which is required to achieve the desired steering angle.
In conventional steering systems, the steering angle sensor which is used to detect the actual steering angle value is arranged in the section on the steering handle side of the steering run, that is in hydraulic servo-assisted steering, between the steering handle and the hydraulic servovalve, and in electrical servo-assisted steering, between the steering handle and the electrical servovalve (e.g., an elastic torque sensor). If, however, there is any relative movement between the control parts of the servovalve, the steering angle set on the steering handle does not match the steering angle which is present at the vehicle steered wheels.
Furthermore, in hydraulically operating servo-assisted steering systems, friction can occur in the servovalve and in the section on the steering handle side of the steering run as well as, in particular, in a gearbox which couples the actuating drive to the steering run. Disturbances are thus caused in the control of the wheel angle setting. Discrepancies between the steering angle of the steering handle and the steering angle of the vehicle steered wheels thus result in an increased control requirement as a result of which, on one hand, the control time is increased and, on the other hand, oscillations are more likely in the control system. In order to allow a predetermined traffic lane to be followed even at relatively high vehicle speeds, the required steering angle values must, however, be set and maintained as exactly and quickly as possible. The known steering systems can thus be used only at relatively low vehicle speeds for the purposes of a traffic-lane following device.